The particular design of muscles and bones in the neck and limbs of the turtle allow that it can draw in its exposed parts such that an attacker can find nothing
but hard shell to bite.

A. allow that it can draw in its exposed parts such that
B. allow it to draw in its exposed parts, and so
C. allows for it to draw in its exposed parts and that
D. allows that it can draw in its exposed parts, and so
E. allows it to draw in its exposed parts, so that

Here the subject is Singular(Design),and not plural.So you require singular verb,hence eliminating others.Finally ans is e,clear in meaning with so that.Whats the Oa?

I htink the Answer is E.Please correct if I am wrong

yes, e is the correct answer.

you should definitely eliminate based on subject-verb agreement first, because such agreement is always clear-cut and unambiguous (there can be no argument about whether a subject or verb is singular or plural). the subject, as has already been correctly asserted on this thread, is 'design'; the subsequent nouns muscles, bones, neck, and limbs are disqualified because they are objects of prepositions. therefore, we need the singular 'allows', so choices a and b are eliminated.

choice d:
'allows that it can...' is an incorrect idiom.
'...and so', while not ungrammatical, departs somewhat from the intended meaning of the original: so that (the original construction) implies a sense of purpose, intent, or design, while and so merely implies consequence, whether designed or arbitrary. for instance:
jim left his parents' back door open so that his friends can come take the liquor --> jim left the door open with this particular nefarious purpose in mind
jim left his parents' back door open, and so his friends can come take the liquor --> jim happened to leave the back door open, an oversight allowing his less straitlaced friends access to the alcohol

choice c:
'allows for it to...' is at best wordy, because the word 'for' serves no grammatical purpose whatsoever, and at worst an incorrect idiom. i of course cannot say which explanation the gmat would give.
'and that' is simply wrong, as there's no other 'that' to which it can be parallel.